Hi,
On 2019-07-31 19:40:09 -0400, Tom Lane wrote:
> Andres Freund <andres@anarazel.de> writes:
> > Unfortunately foreach(ListCell *lc, ...) doesn't work with the current
> > definition. Which I think isn't great, because the large scopes for loop
> > iteration variables imo makes the code harder to reason about.
>
> Yeah, I tried to make that possible when I redid those macros, but
> couldn't find a way :-(. Even granting that we're willing to have
> a different macro for this use-case, it doesn't seem easy, because
> you can only put one <declaration> into the first element of a
> for (;;).
I remember hitting that at one point and annoyed/confused as there that
restriction came from. Probably some grammar difficulties. But still,
odd.
> That makes the other idea (of a foreach-ish macro declaring the
> listcell value variable) problematic, too :-(.
Hm. One way partially around that would be using an anonymous struct
inside the for(). Something like
#define foreach_node(membertype, name, lst) \
for (struct {membertype *node; ListCell *lc; const List *l; int i;} name = {...}; \
...)
which then would allow code like
foreach_node(OpExpr, cur, list)
{
do_something_with_node(cur.node);
foreach_delete_current(cur);
}
That's quite similar to your:
> One idea is that we could do something like
>
> foreach_variant(identifier, list_value)
> {
> type *v = (type *) lfirst_variant(identifier);
> ...
> }
>
> where the "identifier" isn't actually a variable name but just something
> we use to construct the ForEachState variable's name. (The only reason
> we need it is to avoid confusion in cases with nested foreach's.) The
> lfirst_variant macro would fetch the correct value just by looking
> at the ForEachState, so there's no separate ListCell* variable at all.
but would still allow to avoid the variable.
Greetings,
Andres Freund